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Updated NHS Covid-19 app will tell more people to self-isolate


England and Wales’ coronavirus tracing app is set to issue more self-isolation demands as a result of a reduction in the risk threshold it uses, according to officials of the programme.

This comes from improvements of estimates of infectiousness between users and is one of a number of changes being made to the NHS Covid-19 app.

 “We believe lowering the threshold is necessary to reduce the R (reinfection) rate and break the chain of transmission,” the app’s head of product Randeep Sidhu and Gaby Appleton, director of product for Test and Trace, say in a Department for Health and Social Care blogpost.

The risk threshold used by the app will reduce from 900 to 120, but most of this change is due to the addition of an estimate of infectiousness to the app’s algorithm that was made in September.

Sidhu and Appleton write that the app will improve estimates of distance between users by taking advantage of updates by Google and Apple to their application programming interfaces (APIs) to measure the strength of Bluetooth signal between devices.

Along with duration, this is used to decide whether to tell a user to self-isolate if they have been near someone with the virus, but radio waves can bounce off surfaces and generate echoes, making it an unreliable way to estimate distance.

The upgraded APIs attempt to compensate for echoes by considering time data in Bluetooth signals as well as strength to improve their accuracy.

First and only

“Thanks to the hard work of the Test and Trace app team, alongside scientists from the Alan Turing institute, we’re pleased to say that the NHS Covid-19 app is the first and only app globally to have upgraded their technology to utilise this API update (as far as we are aware today),” Sidhu and Appleton say.

NHS Covid-19 will also stop issuing ‘possible exposure’ notifications, generated when a user has been near someone who tests positive for coronavirus without exceeding the risk threshold. Since 13 October the app has issued a follow-up message saying that users do not need to take any action, but the department has decided to remove such messages altogether.

The blogpost claims that with more than 19 million users, NHS Covid-19 downloads are “more than any other European country”. Germany’s Corona Warn app, open source software developed by SAP and Deutsche Telekom for the country’s Robert Koch Institute, had been downloaded 20.3 million times as of 22 October; but given the country’s larger population, the DHSC can claim that NHS Covid-19 has been downloaded by a larger proportion of people in England and Wales.

NHS Covid-19 is set to start working with those offered in other UK nations, the Crown dependencies and Gibraltar from next month. Those serving Scotland, Northern Ireland and Jersey are already able to operate in all three areas.

* NHS Covid-19 does not work properly for users whose smartphones use French, Spanish, Portuguese and other unsupported languages, The Guardian has reported. The app can be used in 10 other languages as well as English and Welsh.

Image from GOV.UK, Open Government Licence v3.0

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